This week, a computational milestone made its debut: a 16-bit microprocessor made up of carbon nanotubes, programmed to greet its makers with the phrase, “Hello, world!”
The breakthrough, described in a paper published Wednesday in the journal Nature, could constitute a crucial step on the path to faster, more energy-efficient electronics.
Standard computer processors typically contain transistors, or electronic switches, that are based in silicon (think “Silicon Valley”). Boosting the speed and power of processors requires building smaller, more efficient transistors—but in recent years, it’s become increasingly apparent that silicon has its limits in terms of scalability. As an alternative, scientists have sought other materials that have the potential to be manipulated on smaller scales. One promising alternative lies in carbon nanotubes: sheets of carbon just one atom thick, rolled up into hardy, lightweight tubes.
Carbon nanotubes are especially attractive to engineers because they’re semiconductors—materials that can either conduct or not conduct electricity, depending on how voltage is applied. This binary on/off property forms the basis of modern computing, and it’s retained by carbon nanotubes on extremely tiny scales, which isn’t true of silicon. In principle, carbon nanotube processors could run three times faster than their silicon counterparts, while consuming just one-third of the energy, study author Max Shulaker told Maria Temming at Science News. That’s close to a 10-fold increase in efficiency.
But carbon nanotubes transistors, which have been around in some form or another for more than two decades, have proven extremely challenging to build. When generating batches of carbon nanotubes, most turn out as semiconductors—but thanks to occasional impurities, about 0.01 percent end up acting like metals, whose conductance can’t be turned off. Another issue is that the tubes tend to glom together when deposited onto computer chips, creating clumps that keep the transistor from working. It’s “like trying to build a brick patio, with a giant boulder in the middle of it,” Shulaker told Temming.
To overcome these hurdles, Shulaker’s team coated the chips with a polymer, then rinsed it off, allowing the problematic clumps to simply wash away. They then purposefully designed the chip’s circuits to work in the presence of impure, conductive nanotubes—a more “if you can’t beat ‘em, join ‘em” technique that the team calls DREAM.
“One of the biggest things that impressed me about this paper was the cleverness of that circuit design,” Michael Arnold, a materials scientist at the University of Wisconsin–Madison not involved in the study, told Temming.
The result was a processor containing 14,000 transistors—the largest and most complex carbon nanotube-based computer chip to date—that was able to execute a program that spat out a salutation: “Hello, World! I am RV16XNano, made from CNTs [carbon nanotubes].” (Outputting the message “Hello, World!” is often one of the first coding exercises that programming students learn.)
“This work takes a big step forward and gets much closer to a commercial chip,” Yanan Sun, a physicist at the Shanghai Jiao Tong University in China, who was not involved in the work, told Elizabeth Gibney at Nature News.
The new chip—intended as more proof-of-concept than commercial product—is far from ready for primetime, however. Its size and capacity still pale in comparison to top-notch silicon processors, which cram many billions of transistors into chips just nanometers across. In its current iteration, the team’s chip performs about as well as a processor that you’d find in a small, homemade robot, Shulaker told Ryan F. Mandelbaum at Gizmodo. There’s also no guarantee the chip’s manufacturing process will scale up once it hits a more factory-like environment.
Even if carbon nanotube chips end up making a bigger splash, that doesn’t necessarily mean curtains for silicon-based processing. The two materials could actually be integrated together in some electronics.
It’s not clear how harmonious that relationship will be. For now, though, the team’s effort represent “a very important milestone in the development of this technology,” Qing Cao, a materials scientist at the University of Illinois at Urbana-Champaign not involved in the study, told Temming.